500 THE GEOLOGY OF MINNESOTA. 



[Gabbro. 



Zoisite fibres and clusters are in a similar manner disseminated (though more 

 frequently isolated) throughout the altered plagioclases. They are distinguishable 

 from the biotite by their low birefringence and their usually less marked fibration. 

 They appear grayish, or blue, in a normal thin section. 



fjt'Hco.re.ne has resulted from the change of the ilmenite. It is strongly cleaved. 

 its lamella? being separated by films of hematite, which also is more or less dissemi- 

 nated through the whole mass of the mineral. By its increasing abundance, in the 

 direction toward the original grain of ilmenite, the transparency of the leucoxene is 

 lost by degrees, and it apparently blends with the original ilmenite. The coarse 

 cleavage visible is probably due to the original zoned structure of the ilmenite. 



Two sections. 



A</<: Cabotian. N. H. w. 



No. 700. GABHHO (willi 



Same locality as No. G95. The ore (No. 005) lies on this rock, which seems to have furnished the rounded 

 masses of No. 698 inclosed in the ore. 



Ref. Annual Report, x, pages 81, 83; Bulletin ii, page 93, plate III, figure 2; Bulletin vi, page 136. 



A rather fine-grained, dark-gray, granular rock, composed of feldspar and 

 a dark mineral or minerals. 



Mic. M. E. Wadsworth's description of this rock is as follows:* 



"Is a compact, dark-grayish, crystalline rock, composed of ilinllayr, nlirini 1 

 J'l'lilsjKir and nnHjnctitr. Considerable of the secondary lint He is found associated with 

 the magnetite and diallage. This section has its minerals somewhat altered, the 

 magnetite dust and grains being quite abundant in the diallage, while a chlorite- 

 like vein traverses the section. 



" Plate III, figure 2, shows the general structure of the rock with its brownish 

 diallage, yellowish altered olivines, black magnetite, reddish-brown biotite in the 

 diallage and bordering the magnetite, and colorless feldspar." 



The olivine is frequently in somewhat rounded grains and is in general earlier 

 than the augite (diallage) and feldspar. Some of the augite is later than some of the 

 feldspar, which in places has a tendency to a lath-shaped development. The augite 

 sometimes shows twin lamellae parallel to UK). The feldspar is commonly twinned 

 according to the albite law. Equal extinction angles in sections normal to 010 run 

 up to 32, and a cleavage chip parallel to 010 gives an extinction angle of 22. Tho 

 feldspar is therefore l<ilr<i<lorit<'. One section examined. 



Ayr. Cabotian. r. s. o. 



No. Tdl. <l. \iaiRO. 



Near the rent re of S. }^ see. M, T. K> '1 \\ .: north shore of Ma.vhew hike. 

 Annual Hrport. x. pair's 7S, Si!, HI; I'.ulletin ii, ]>a<,'e 7'2. 



*tSultetin ii. p. 9i. 



